1. Field of the Invention
The invention relates to a system and method to operate an internal combustion engine to provide unburned fuel and air into the exhaust aftertreatment device to improve the conversion efficiency of the exhaust aftertreatment device.
2. Background of the Invention
A factor in achieving low exhaust emissions from internal combustion engines, at the current state of the art, is to bring the exhaust aftertreatment device to its operating temperature as rapidly as possible after initiating engine operation. It has long been recognized that by introducing some unburned fuel and air into the exhaust duct of the engine at a location upstream of the exhaust aftertreatment device that the fuel and air react in the exhaust aftertreatment device creating an exotherm, thereby quickly raising the temperature of the exhaust aftertreatment device.
A common method to provide fuel and air to the exhaust aftertreatment device is described in U.S. Pat. No. 5,410,872, which is assigned to the assignee of the present invention. The air and fuel delivered to the combustion chamber of the engine is rich, i.e., contains excess fuel. Consequently, the engine exhaust contains unburned and partially burned fuel. Secondary air is introduced into the exhaust duct by an air pump. The incompletely burned fuel and secondary air mix and are introduced into the exhaust aftertreatment device in which oxidation occurs, creating an exotherm.
A problem with prior art approaches is in metering the fuel, primary air, and secondary air to provide the desired mixture and flow rates to the combustion chamber and to the exhaust aftertreatment device. The constraints are: providing a stoichiometric mixture (air to fuel ratio such that if reacted to completion, fuel and oxygen are completely consumed) to the exhaust aftertreatment device, providing sufficient unburned mixture to the exhaust aftertreatment device to provide the desired temperature rise in the exhaust aftertreatment device, and providing a rich, but combustible mixture, to the combustion chamber. This combination of constraints presents a complicated control task.
Another difficulty with prior art is the hardware required to accomplish the task: air pump, air lines, switches, metering devices, and others, which add weight, cost, additional plumbing, etc.
The inventor of the present invention has recognized a method to provide unburned fuel and air to the exhaust aftertreatment device overcoming metering problems and relying on existing hardware.
The above disadvantages are overcome by a method for operating a multi-cylinder internal combustion engine by providing an amount of air and fuel to a cylinder of the engine. The air and fuel are compressed. During compression, an exhaust valve of the cylinder is opened releasing a portion of the air and fuel into the exhaust aftertreatment device. In this way, the temperature of the exhaust aftertreatment device is increased.
Preferably, a system is disclosed for providing fuel and air to an exhaust aftertreatment device of a reciprocating multi-cylinder internal combustion engine which includes an exhaust valve coupled to a cylinder of the engine, capable of being actuated during a compression stroke of the engine, and an engine controller connected to the engine and the exhaust valve to actuate the exhaust valve during the compression stroke to release a portion of the contents of the cylinder into the exhaust aftertreatment device.
An advantage of the present invention utilized with an engine having exhaust valves that can be opened during the compression stroke, is that fuel and air from the combustion chamber can be released into the exhaust system without a separate air pump, additional plumbing, and valves of prior art approaches.
An advantage of the present invention is that the fuel and air are metered by existing hardware and well known strategies. That is, no control strategy need be developed to meter the fuel, primary air, and secondary air, as is the case in prior art. Specifically in the present invention, the air-fuel ratio may be controlled open loop based on a measure of airflow to the engine and controlling fuel pulse width or closed loop based on a signal from an exhaust gas oxygen sensor according to the present invention.
A further advantage of the present invention is that the air and fuel delivered to the combustion chamber may be in stoichiometric proportion thereby overcoming the potential of a rich misfire in the combustion chamber as may occur in the prior art.
Another advantage is that the present invention may be used in a diesel engine, or other engine with low exhaust temperatures such as homogeneous charge compression ignition engines, to maintain a high enough exhaust temperature in an exhaust aftertreatment device for high conversion efficiency.
The above advantages and other advantages, objects, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.